The present invention relates to an electromechanical element, a driving method of the electromechanical element and an electronic equipment provided with the electromechanical element.
An electromechanical element has many application fields such as wireless communication, light, acceleration sensor or biotechnology. Among others, the electromechanical element can be applied to a switch or filter for radio apparatus.
Along with the spread of the information communication equipment such as a wireless communication terminal, the frequency band used in the communication becomes increasingly more wide from hundreds MHz for portable telephone to a few GHz for wireless LAN. In a current situation, the terminals for supporting various communication methods are independently employed, but it is desired to realize a small wireless terminal capable of supporting various communication methods at one wireless terminal in the future. Although the number of passive components such as a switch contained in a housing of the terminal is expected to increase, it is desired to reduce the size of passive components.
In particular, the research and development of an RF-MEMS switch fabricated by an MEMS (Micro Electro Mechanical Systems) technology have been actively made. The RF-MEMS switch is the switch that mechanically switches the signal propagation path by moving a minute movable electrode. The advantage is that it has a small device size and excellent high frequency characteristics such as extremely low loss and high isolation. Since the switch can be manufactured through a process having good affinity with an RF-IC, it can be contained in the RF-IC. Therefore, the MEMS technology is expected as the technology greatly contributing to size reduction of a wireless portion.
The related RF-MEMS switch is a mechanical switch that switches the signal propagation path by contacting or separating a membrane or bar-like movable body, which is supported at both ends or only one end, with or from the electrode. Most of the driving force sources for the membrane or movable body employ electrostatic force, but other sources employing magnetic force have been already announced.
Conventionally, a minute switch as large as about several hundreds μm is well known as disclosed in a non-patent document 1. This switch is made by forming a signal line for passing the high frequency signal on the membrane and providing a control electrode directly below the signal line. If a dc potential is applied to the control electrode, the membrane is attracted toward the control electrode due to an electrostatic attracting force, and flexed to make contact with the ground electrode formed on the substrate, so that the signal line formed on the membrane is short-circuited, and the signal flowing through the signal line is attenuated and cut off. On the contrary, if the dc current is not applied to the control electrode, the membrane is not flexed, so that the signal flowing through the signal line on the membrane is not lost from the ground electrode but passes through the switch.
In propagating the signal by resistance coupling, it is required to form an excellent metal contact point that is stable and has low resistance. A technique for forming the excellent metal contact point owing to the effect (mechanical cleaning effect) of breaking through an oxide film on the surface of a fixed electrode by developing a pushing force for a movable electrode into the fixed electrode contacted by curving the movable electrode has been reported in patent document 1.    [Non-patent document 1] J. B. Muldavin and G. M. Rebeiz, IEEE Microwave Wireless Compon. Lett., vol. 11, pp. 334-336, August 2001    Patent document 1] WO01/88933
However, at present, when the excellent contact point of the electromechanical switch is formed, there occurs a sticking phenomenon (stiction) in which the contact point is sticky and difficult to release (separate). As a result, there is a problem that the reliability is decreased. Also, there is a problematical phenomenon that the contact point is deteriorated due to continuous driving or propagation of high power signal, thereby, the resistance value is increased.